Disclosed is a device for introducing a medium into the body, in particular an annular groove-positioned in vitro infusion catheter stagnation switch, composed of the following main points: comprising a joint base (3) and a joint body (2), wherein the front end of the joint base (3) is connected to the infusion catheter (4) or the infusion member on the infusion line, the rear end of the joint body (2) is connected to the infusion catheter (4), and the front part of the joint body (2) is inserted into the lumen of the joint base (3) in a telescopically sliding manner. It has the advantages that the means for preventing and eliminating negative pressure is simple, effective and easy to operate. The intervention of a cut-off valve (7) cuts off the infusion channel while also eliminating negative pressure. This is not only more hygienic and safe, but also ensures that the joint body must be pushed back before performing another infusion (otherwise the cut-off valve is cut-off and the infusion cannot be completed), so that after the infusion has been completed, there is an advancing space for the joint body (2) to eliminate negative pressure. The cut-off valve (7) also has a use as a flow regulator valve for pushing the joint body (2) forwards to gradually cut off. Furthermore, the provision of an annular groove (31) and a projection (25) ensures that the joint body (2) can be effectively positioned when advanced, and that it is not easy to actuate the joint body (2) due to malfunction.